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About a month ago, I wrote about spaying and neutering cats. My conclusion was that there was no simple answer to the question of when or even whether cats should be altered. A commenter accused me of obfuscation and derided me for not providing a “clear-cut thesis.” Her idea of obfuscation on this cat-health issue was my idea of honesty — there was no clear-cut thesis because providing one was impossible amongst so much contradictory data.

Today’s column touches on an equally difficult concept regarding cats and health: vaccines. If you are looking for a simple answer, prepare to be disappointed. Vaccines are complicated, and no honest and knowledgeable person can make them simple. In fact, in the realm of vaccines, claims of simplicity are a sign of either ignorance or deceit. Anyone who doesn’t acknowledge that the matter is complex either doesn’t understand it or is lying to push an agenda.

About 25 years ago, vaccines were simple. Give every cat every shot every year and you couldn’t go wrong. Or so the thinking went.

But in 1991, investigators at the University of Pennsylvania School of Veterinary Medicine noticed that cats were developing aggressive cancers called sarcomas between their shoulders. This area happened to be the spot where cats most frequently received vaccines. Further studies established a probable causal link between vaccines and these so-called vaccine-associated sarcomas.

This discovery shocked the world of veterinary medicine. It caused a great deal of soul searching in many vets, and it caused a significant rift between the soul-searching vets and the vets (most of whom were older, and most of whom are no longer practicing) who frankly didn’t care. I know more than one colleague who quit a job in part because her boss insisted that she recommend every vaccine for every cat every year.

Above all, remember that the subject is complex. I have twice seen leading experts on vaccination nearly come to fisticuffs when debating vaccine frequency in cats. But is there some way to tease out some clarity in this situation? Although there is no simple answer when it comes to vaccines and their frequency, there are some guiding principles that can be used.

1. Some vaccines are more important than others

In my mind, there are really only three vaccines that should be considered for most cats: rabies, FVRCP (which protects against deadly panleukopenia, and also may help to prevent certain respiratory diseases), and leukemia. The coronavirus vaccine (which may offer some protection against feline infectious peritonitis) may be considered for cats in rare circumstances. Other vaccines, such as those against Bordetella, FIV/feline AIDS, and ringworm (which is off the market at this time) appear to offer minimal benefit or efficacy.

2. Even the important vaccines might not be appropriate for all cats

Elderly cats are not at significant risk of panleukopenia. Indoor cats are not at risk of feline leukemia.

3. Timetables have changed

After a lengthy battle, the proponents of annual vaccination for cats are in full retreat. Most reputable organizations (such as the American Animal Hospital Association and many vet schools) recommend vaccination every three years at most. However, this is just a guideline; actual vaccination schedules must be individually tailored.

4. Younger cats need vaccines more than older cats

This is true despite the fact that over-vaccination can have negative consequences. Unvaccinated kittens succumb to panleukopenia at horrifying rates, but panleukopenia is essentially unheard of in 18-year-old cats. Similarly, leukemia is a much greater threat in younger cats than in older ones.

5. Outdoor cats need more vaccines than indoor cats

This applies most specifically to the vaccine for feline leukemia virus. The simplest way to avoid this dreaded disease is to keep your cat indoors. If your cat goes outside, then the benefits of leukemia vaccination likely outweigh the risks, especially when he is young.

6. Be wary of non-adjuvanted vaccines

One company has produced vaccines that do not contain a product called “adjuvant.” The purpose of adjuvant is to increase the immune system’s response to vaccines. Some people have theorized that adjuvant contributes to the development of sarcomas. However, I have never seen any solid evidence that non-adjuvanted vaccines are safer. A recent major study in the Journal of the American Veterinary Medical Association discredited the notion that adjuvant plays a significant role in the development of sarcomas. And, to make matters worse, the non-adjuvanted vaccines must be given annually rather than triennially in order to fulfill the recommendations of the manufacturer. I am very reluctant to believe that these vaccines are three times less likely to cause sarcomas. Unfortunately that is the threshold they need to cross, since they’re given three times as often as the annual adjuvanted vaccines.

7. Rabies is a special case

The vaccine is required by law for cats in some jurisdictions but not in others. Rabies also is zoonotic — this means it can spread from cats to humans. And it has the dubious distinction of being the deadliest virus on earth. The rabies vaccine unfortunately appears to cause sarcomas at a higher rate than the FVRCP. These many facets of rabies vaccination must be considered whenever a cat is evaluated for vaccines. Parents of young children may elect to vaccinate their cats for rabies while single adults may not. Cats who go outside and hunt will be at greater risk of exposure to rabies; the vaccine should be considered more seriously in these individuals. However, even indoor cats may be exposed to rabies. Odd though it may sound, rabid bats have been known to expose indoor-only cats to the terrible virus after they fly through windows or down chimneys.

8. Sarcomas are a rare but real threat

Although rabies and leukemia vaccines appear to be most likely to cause sarcomas, no vaccine — in fact, no injection — is free of risk. The risk of sarcomas, as well as other rare but possible negative consequences of vaccination, must be weighed against the benefits of any vaccine.

So, what should you do? I have two suggestions. First, keep your cat indoors. This will reduce the need for leukemia vaccination and will make the matter simpler. Second, find a good vet. Good vets will be happy to take the time to discuss the nuances of these issues and to consider your cat’s unique circumstances before making vaccine recommendations. Beware of anyone who tries to make this matter sound simple or clear-cut.

In the end, remember that it is you, and you alone, who must make the decisions. Your best bet is to be as knowledgeable as possible before making them. However, I have a word of warning for you: The more you know about vaccines, the more complicated and difficult this subject becomes.

Got a question for Dr. Barchas? Ask our vet in the comments below and your topic might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)